In general, in telephone communication allowing direct conversation with a destination, there is an advantage in that detailed information can be immediately exchanged, but both of a calling person and a called person have to be ready for telephone call at the same time. For the above technique for efficiently establishing a telephone call which is initiated upon intentions indicated by both of a calling user and a called user, various suggestions have been made so far.
For example, PTL 1 discloses a technique in which a receiving call condition is set in advance by a called side for the purpose of allowing both of a calling side and the called side to reliably establish a highly important telephone call, and only when an outgoing call condition of the calling side and the receiving call condition of the called side match each other, the receiving call is automatically received.
PTL 2 and PTL 3 disclose techniques in which, when the called side is absent, a notification is sent as soon as the called side becomes ready to respond the call, or a notification is sent to indicate when the called side becomes available to respond the call, for the purpose of preventing waste of a time which is required to establish a telephone call due to the absence of one of them. PTL 4 discloses a technique in which a sending party of an e-mail writes, in the text body of the e-mail, to ask for making a telephone call at a specified time, so that the receiving party automatically analyzes the e-mail and makes a call to the sending party.